Please look into this :)

Maybe it doesn't work, because you are using a non-official map.

There might be a few (not yet official) structures and buildings, that don't support weapon deployment.

The developers (for now) only support issues of the official content (ArmA 3 maps, structures, weapons, functions, scripts, vehicles etc.).

@Koala that's not fairly accurate, the developers said they're making weapon resting and bipods so that it will work with all environments and custom maps.
I remember that part as it really seemed like an awesome feature.
The engine scans for all objects and structures that you can rest your weapon on so if the object only has texture but no real structure then that's possibly the reason.

So if you can walk through it then you probably can't rest on it, have you tried that? Also did you try to bring yourself higher a notch with LCTRL+W?

Improved title, forwarded.

This ticket can be closed as issue is fixed.

Thanks.